The development of the EDVAC computer system of 1948 is often cited as the beginning of the computer era. Since that time, computer systems have evolved into extremely complicated devices. To be sure, today's computers are more sophisticated than early systems such as the EDVAC. Fundamentally speaking, however, the most basic requirements levied upon computer systems have not changed. Now, as in the past, a computer system's job is to access, manipulate, and store information. This fact is true regardless of the type or vintage of computer system.
High-availability (HA) computers are the result of a system design approach focused on maximizing system readiness, or conversely, on minimizing unplanned downtime. The key tools used by designers of these systems are redundancy and spare capacity. To facilitate redundancy and capacity, HA systems frequently utilize arrays of direct access storage devices (DASD), such as hard disk drives, organized and used in a redundant fashion. This allows each DASD device to be replaced without losing stored data. These arrays may often be combined with hot-swapping technology, which provides the ability to remove and replace hardware without interrupting the operation of the system/server. In this way, DASD's can be replaced without requiring the HA computer to be shutdown.